Devices that transmit signals between electrically isolated input and output circuits are of considerable commercial importance. For many purposes, electromechanical relays or isolation transformers provide adequate electrical isolation between the input and output circuits. However, these devices suffer the drawbacks of large physical size and slow response time in comparison with solid state circuitry.
To overcome these drawbacks, as well as for other reasons, devices, commonly referred to as opto-isolators, that use optical coupling to link the electrical input and output circuits have been developed. A light source, commonly a light-emitting diode (LED), located in the input circuit and a photodetector, located in the output circuit and optically coupled to the light source, are the essential elements of the device. Current in the input circuit which passes through the LED causes the LED to emit light. Some of this light is received by the photodetector and causes an electrical current to be generated or controlled in the output circuit. Opto-isolators offer complete electrical isolation between the circuits and small size.
For many purposes, a bilateral opto-isolator, that is, an opto-isolator whose operation does not depend upon the polarity of the voltage applied across the output circuit, is desirable. One promising approach to the construction of bilateral opto-isolators uses two series connected illuminated photodiode arrays to generate a voltage which controls the current through a field effect transistor in the output circuit. One array is connected between the gate and drain electrodes, and the other is connected between the gate and source electrodes. Such devices have the advantages of not requiring separate voltage sources to reverse bias the gate-source and gate-drain junctions of the field effect transistor and of not requiring the field effect transistor to be photosensitive. Furthermore, these bilateral devices have required two photodiode arrays and a single field effect transistor. Another promising approach uses a single photodiode array and two field effect transistors to achieve bilateral operation.
Other voltage controlled devices such as the recently developed gated diode switch (GDS) also exist. Gated diode switches can be fabricated in embodiments that will block voltages as large as 500 volts and carry currents larger than one ampere. A bilateral opto-isolator requiring only a single photodiode array to control the current through a voltage controlled element or elements, such as a field effect transistor or two antiparallel connected gated diode switches, would be desirable.